1. Field of the Invention
The present invention relates generally to the field of optics and, in particular, to a bifocal corneal contact lens.
2. Description of the Related Art
Although refracting contact lenses have been in existence for nearly 100 years, they have generally been unsuccessful when adapted for use as a bifocal lens. Bifocal contact lenses have been commercially available for the past 30 years but, for various reasons have not proven to be successful. Generally, these lenses tend to have either good distance vision with poor reading vision, or good reading vision with poor distance vision. Part of the reason for this is the delineation between near and far vision portions of the lens created by polishing.
One type of best known bifocal contact lens is referred to as the crescent seg fused bifocal. A segment of higher index plastic is fused into a lower index plastic matrix to create a reading section. This reading section is held in correct position on the eye by the use of prism for ballast. However, edema has been induced by such lenses because of their use of PMMA plastic. Thus, the wearing time is diminished, and the lenses are also uncomfortable due to the creation of a prism and the weight of the lens.
Other attempts to create a viable bifocal contact lens have led to the use of a lens which has concentric far and near portions. Referring to FIGS. 1 and 2, a contact lens 10 has two concentric portions, a far distance portion 12 which is circular in shape, and a near distance portion 14 which is annularly shaped around the central circular portion.
In the manufacturing process, the far and near vision portions are cut and polished to achieve the required refraction. The polishing occurs on the anterior surface 16 of the lens 10 while rotating same and applying a polishing tool and polishing fluid to the anterior surface 16.
The polishing process leaves the anterior surface with different radii of curvature for the different vision zones. As seen in FIG. 1, the far distance portion 12 has a flatter curvature than the near distance portion 14. The difference in curvature results in the formation of a sharp, annular junction 20 where the curvature radius changes between the two vision portions. This junction creates two problems. First, it creates a line between the near and far vision portions which, when the lens moves on the eye, creates an image jump or double vision. Secondly, the junction 20 creates a source of irritation for the user because of the non-curvilinearity of the anterior surface 16.